I. Field of the Invention
The present invention relates generally to power gain control for a power amplifier and particularly to a wireless communication device, such as a CDMA wireless phone.
II. Description of the Related Art
In many electronic environments, such as most hand-held communication systems including code-division-multiple-access (CDMA) or any form of time-division-multiple access (TDMA) technology, RF power output from a mobile unit varies in large dynamic ranges. In a CDMA radiotelephone system, multiple signals are transmitted simultaneously at the same frequency. The signals are spread with different digital codes, thus allowing detection of the desired signal while the unintended signals appear as noise or interference to the receiver. Spread spectrum systems can tolerate some interference, and the interference added by each new mobile station increases the overall interference in each cell site. Each mobile station introduces a unique level of interference, which depends on its received power level at the cell site.
The CDMA system uses power control to minimize mutual interference. A precise power control is critical to avoid excessive transmitter signal power that is responsible for contributing to the overall interference of the system. Power of the individual mobile stations varies with the distance between the mobile station and the base station and the number of other subscriber mobile stations in that base station or sector.
In a typical hand-held wireless unit, the power amplifier is biased class AB to reduce power consumption during periods of low transmit power, but power continues to be consumed. Typically an isolator is used to isolate the power amplifier from the effects of load impedance in subsequent stages. One method to avoid continuous battery draw is to employ a means to bypass the amplifier with switches, and then remove DC power from the Amplifier. This method is illustrated in FIG. 7. A power amplifier circuit 8 is shown with a power amplifier 32 and an isolator 55. An RF input 12 having an RF-signal to be amplified is connected to a pole of a first switch 20. When the amplifier is on, the switch 20 connects the RF-input 12, via path 28, to an input of power amplifier 32. The RF-signal is amplified and output to the isolator 55, and then transmitted through the second switch 42 to the RF-output 54 of the power amplifier circuit 8. To bypass the power amplifier 32, the first switch 20 connects the RF-input 12 to the bypass path 30 and the second switch 42 transmits the signal to the RF-output 54. The drawback of this technique is that the amplifier must overcome the added switching loss during times that higher transmit power is required. This can tend to cancel the benefits of bypassing. Furthermore, using a switch and an isolator requires more power to operate and is more costly to build.
FIG. 6 illustrates a prior art power amplifier circuit. An analog signal is fed from a driver amplifier 280 through a band pass filter 298 to a first switch 20. The switch 20 alternates between a bypass path 30 and an amplifier path 28, wherein a power amplifier 32 amplifies the signal. A second switch 42 connects the analog signal from either the bypass path 30 or the output of amplifier 32 to a circulator 55, which routes the signal to the RF-output port 54.